(1) Field of the Invention
The present invention lies in the area of aircraft flight instruments. More specifically, the invention relates to a flight instrument that displays a variable rotational speed of a main rotor of a rotary-wing aircraft, and also relates to a flight instrument procedure for displaying such a speed.
(2) Description of Related Art
A rotary-wing aircraft usually includes at least one main rotor that is rotatively driven by a power plant on board the aircraft. Thus, the main rotor supports the aircraft and provides its propulsion. Such an aircraft may also include an anti-torque rear rotor, or, for example, one or two propellers.
In particular, such a rotary-wing aircraft is piloted through the monitoring of numerous instruments that represent the operation of the power plant and of the aircraft. In fact, numerous mechanical, physical, and aerodynamic limitations must be taken into consideration by the pilot while the aircraft is being flown.
In particular, one instrument indicates, in real time, the rotational speed of the main rotor of the aircraft. This rotational speed of the main rotor of the aircraft is often designated by the abbreviation “NR”.
In particular, the main rotor provides the lift that is necessary in order to support the aircraft. This lift provided by the main rotor is directly linked to the rotational speed of this main rotor. Therefore, control of this rotational speed of the main rotor of the aircraft is essential in order to ensure this lift and, consequently, to ensure the flight of the aircraft.
In contemporary aircraft, the rotational speed of the main rotor is often essentially constant. However, this rotational speed may vary over a limited range above and below a constant value that constitutes a fixed setpoint for the rotational speed of the main rotor. This rotational speed may vary depending on flight conditions, such as the altitude of the aircraft or even the path followed by the aircraft.
For example, this limited range may represent up to 7% of this fixed setpoint, with the speeds that constitute variations of this rotational speed of the main rotor (that is, the acceleration or the deceleration of the main rotor in rotation) being on the order of 1% of this fixed setpoint per second. The term “range” is understood as referring to an interval of values that may be taken on by the rotational speed of the main rotor of the aircraft.
For example, for a rotational speed of the main rotor on the order of 300 revolutions per minute (300 rpm), the variation speed of this rotational speed is on the order of 3 revolutions per second (3 rpm/sec).
Thus, for this type of aircraft and in certain phases of flight, the control of this rotational speed of the main rotor is based on sound. In fact, the pilot typically knows, through habituation, the sound emitted by the main rotor when it is rotating at the fixed setpoint. Indeed, thanks to his experience, the pilot is capable of confirming by ear that this rotational speed of the main rotor is in fact constant and essentially compliant with the fixed setpoint.
The instrument that indicates this rotational speed of the main rotor makes it possible to control the value of this rotational speed of the main rotor of the aircraft, and is used essentially by the pilot during flight phases for which this rotational speed of the main rotor varies.
This instrument is also used during specific flight phases, such as during flight under autorotation or in the event of an engine failure during stationary flight.
In most aircraft this instrument is an analog instrument, and is graduated in terms of percentages of the fixed setpoint for the rotational speed of the main rotor, with this fixed setpoint corresponding to a graduation mark of “100%”. This instrument also includes indications that correspond to fixed rotational speed limits that must not be exceeded. In fact, if these fixed limits are exceeded, damaging effects on the mechanical power-transmission train may occur, or a risk of engine flameout may arise, for example, due to excessively sudden deceleration.
In certain aircraft, this instrument may also indicate the rotational speed of the free turbine of each turboshaft engine that drives the main rotor. In fact, by means of their free turbine, these turboshaft engines drive the main power transmission gearbox, and, consequently, the main rotor. Indeed, there is a constant ratio of proportionality between the rotational speed of each free turbine and the rotational speed of the main rotor.
In recently manufactured aircraft this instrument has been replaced by a digital display that indicates, in numerals, the percentage of the fixed setpoint that constitutes the current value of the rotational speed of the main rotor. An analog instrument may still be present; however, in such a case it constitutes a back-up instrument in the event of a failure of the digital display.
Nevertheless, the rotational speed of the main rotor of a rotary-wing aircraft can be caused to vary voluntarily and continuously over an expanded range that may represent, for example, as much as 15 or 20% of an average rotational speed of this main rotor. Therefore, the setpoint that this rotational speed of the main rotor must maintain is no longer fixed, but instead is variable, and may change continuously within this expanded range during the flight of the aircraft.
Such variations in the rotational speed of the main rotor, which may occur with major speed variations, make it possible to obtain several improvements in the operation of the aircraft, including, in particular, a reduction in the noise generated by the main rotor, as well as an increase in the maneuverability of the aircraft and in its performance.
In the specific case of a hybrid aircraft that flies with a high forward speed, the rotational speed of the main rotor must be reduced in order to prevent excessive speed in relation to the air at the end of each blade of this main rotor and, in particular, to prevent the said speed from exceeding the speed of sound.
For example, the variation speed of the rotational speed of the main rotor is on the order of 5% of this rotational speed of the main rotor per second.
Conversely, because the setpoint for the rotational speed of the main rotor is variable, the rotational speed of the main rotor may be equal to each value within the expanded range, depending on the flight conditions.
Indeed, the pilot of the aircraft must make sure that this rotational speed of the main rotor always remains essentially equal to the variable setpoint—a task that requires his attention even when the flight status is normal.
Furthermore, the instrument that currently indicates this rotational speed of the main rotor does not provide effective assistance to the pilot, because the variable setpoint does not appear on this instrument. Obviously, this variable setpoint could be added to this instrument. However, because the setpoint can vary continuously, such an instrument would make heavy demands on the attention of the pilot of the aircraft in order to identify the variable setpoint for the current rotational speed of the main rotor.
Furthermore, because this rotational speed of the main rotor varies continuously, the sound emitted by the main rotor changes routinely during the course of the flight. Similarly, even if the pilot continues to monitor the rotational speed of the main rotor by ear, the pilot can no longer maintain accurate and effective sound-based control of this rotational speed of the main rotor (that is, control based on the frequency of the sound emitted by the rotor). Thus, the pilot requires an instrument that accurately indicates this rotational speed of the main rotor of the aircraft.
Consequently, the use of a variable setpoint for the rotational speed of the main rotor of an aircraft requires the use of an instrument that indicates to the pilot, in a clear and simple manner, both the variable setpoint and the current rotational speed of the main rotor. Furthermore, the use and the broader dissemination, in the near future, of such a variable setpoint may be confusing or disconcerting to pilots of rotary-wing aircraft, because it calls into question certain customary practices for flying such an aircraft in relation to its traditional use.
Thus, the presence of such an instrument should be included among the key stages of the acceptance of this variable setpoint by the pilots. Conversely, the use of inappropriate instruments may limit the range covered by the rotational speed of the main rotor and/or its dynamic operation, thereby limiting the improvements contributed by the use of this variable setpoint in terms of the operation of the aircraft per se.
Document FR2756256 is known, which describes a power-margin indicator for a rotary-wing aircraft. This indicator shows the current value of the collective pitch of the blades of the main rotor of the aircraft, as well as an available power margin that is represented in terms of a collective pitch margin. This power margin is calculated in terms of the technical limitations of the motors and of the main power transmission gearbox of this aircraft.
Furthermore, document WO97/42466 describes a variable parameter display whose background changes depending on the operating mode and/or the circumstances. The graduation marks are fixed, but the display of these graduations, as well as the color of the marks, may vary, particularly as a function of the circumstances. The parameter that is displayed may consist, for example, of the rotational speed of the main rotor of a rotary-wing aircraft.
Meanwhile, document EP2402716 describes a system that is capable of drawing the attention of the pilot of an aircraft to a particular indicator. For this purpose, the system may zoom in on a specific portion of this indicator, for example, when this indicator approaches a setpoint value or a limit value.
Document WO2006/081334 is also known, which describes a power indicator for a rotary-wing aircraft. In particular, this power indicator may provide information about the rotational speed of the main rotor of this aircraft. It displays, in columns on a graph, the setpoint for this rotational speed of the main rotor; the current value of this rotational speed of the main rotor; and minimum and maximum values for this rotational speed of the main rotor.
Moreover, document FR2943131 describes a flight indicator that shows, on a graduated moving scale, information about the current collective pitches, the limit values, and the target values for the main-rotor blades.
Last, document FR2946322 describes a flight instrument for a hybrid helicopter that makes it possible to display a maximum average pitch that is applicable to the propellers of this hybrid helicopter.